The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus of tube current modulation for radiographic imaging, e.g. computed tomography (CT).
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray- detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
As described above, CT imaging is an imaging modality predicated upon the projection of radiographic imaging, e.g. x-rays, and reconstructing an image of the subject based on the subject's attenuation of the projected x-rays. Generally, driving an x-ray source at higher current levels produces images with less noise. On the other hand, extremely low x-ray tube current levels can cause severe artifacts in the reconstructed image. X-ray tube current may be characterized as being directly related to the amount of radiographic energy received by the subject, i.e. patient dose. As such, as x-ray tube current increases, so does the radiation dose received by the subject. While higher x-ray tube current levels result in less noisy images, higher tube current levels expose the subject to increased x-ray dose. Therefore, in establishing an imaging protocol for a given subject, a trade-off must be made between tube current and subject dose. Ideally, it is preferred to use the minimum radiation dose necessary to generate a diagnostically valuable image.
A number of techniques have been developed to determine a tube current modulation profile that achieves the two desire objectives: (1) diagnostically valuable images; and (2) minimum radiation exposure to the subject. A number of these techniques are predicted upon the acquisition and analysis of scout scan data to shape a tube current modulation profile that satisfies the above objectives. Notwithstanding the advancements achieved by these known imaging techniques, it has been shown that over-exposure as well as under-exposure of radiation can still be problematic and therefore expose the subject to unnecessary radiation or result in a noisy image that therefore requires re-scanning of the subject.
A number of tube current modulation techniques have been developed to enhance waveform shaping and x-ray generation using projection data from one or more scout scans. These techniques assume that the tube current modulation profile is symmetrical throughout a single gantry rotation cycle. It has been shown, however, that the ideal tube current modulation waveform may not be symmetrical. That is, these known techniques to determine tube current modulation fail to account for the asymmetry of the ideal modulating waveform. This asymmetry results in the subject being over-exposed or under-exposed to radiation depending upon the diagnostic objectives of the scan.
It would therefore be desirable to design a method and apparatus for tube current modulation that accounts for the asymmetry of an ideal tube current modulating profile.